


Monster in Me

by FullOfBoredom



Category: Undertale (Video Game)
Genre: Cherryberry - Freeform, Doggy Style, M/M, Monster sex, Multiple Orgasms, Outdoor Sex, Overstimulation, Size Difference
Language: English
Status: Completed
Published: 2021-01-24
Updated: 2021-01-24
Packaged: 2021-03-16 02:29:35
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,150
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28948956
Author URL: https://archiveofourown.org/users/FullOfBoredom/pseuds/FullOfBoredom
Summary: Blue's between a rock and a hard place. It's more enjoyable than it sounds.
Relationships: Sans/Sans (Undertale)
Comments: 4
Kudos: 47





	Monster in Me

**Author's Note:**

> Done as part of Kinktober.

“Oh no.” Blue looked over at his boyfriend Red, currently panting from magical exhaustion but still forcing himself onward. He had to stop the encroaching feral creatures from the forest, Blue and Red agreeing to hold the line here until the others could fend off the main threat.

Blue could feel his limit too, but held back from pushing too hard or he’d…change.

All the skelebros had discovered back in their own universes that upon pushing too hard, pulling more than they had, they’d transform into gaster beasts, large skeleton creatures with wide hinged heads and long bony tails that left their bodies uncontrollable. They couldn’t turn back until their magic ran out again. Which considering they had to trap the beast so it couldn’t hurt people, it took DAYS. 

Since the convergence, it had happened to all of them at least once. Besides Red.

Red put in as much effort as the others, quite frequently in fact, but he never turned. Red often dealt with gaster beasts the best as well. He credited his brother making for a wily monster, difficult to trap even out of his mind. Blue, meanwhile, was pretty bad at catching them, so the prospect of facing Red that way was terrifying.

Red’s strength was waning, the shift crawling up his bones. The creatures diverted over to Blue. 

“Fuck!” Red reached out for him, but the transformation had started and he was powerless to stop it. Blue sent out a few waves of bones to keep them in check, but there were too many, Blue couldn’t summon the energy, and if they both lost it, this area would be decimated. Blue stumbled back into a tree.

“AHHH!” Blue covered his face and prayed, hiding his face behind his arms when he saw the horde leap at him.

“GRRRRRRAAAARRRRR!” A tail whipped the creatures back. The red beast pounced on the attackers, swiping with his claws, creating dust wherever he stomped. After a good frontal assault, the creatures retreated into the trees. Then the giant head swivelled around.

Blue froze under the gaze. It still had Red’s eyes. He wasn’t prepared for that.

“I love you, even if you kill me.” Blue didn’t bother to block, just hyperventilating as Red’s giant face creeped closer and closer. Red’s breath puffed into his face before he stopped, Blue kicking at the dirt to get as close to the tree as possible, finally turning his head and closing his eyes. Then he felt a lick across his skull.

Blue finally took in Red’s beastly body. It looked relaxed, casual. It dropped it’s head in Blue’s lap. It’s throat let out a low gravelly hum, eyes looking up at Blue’s face.

Blue hesitantly laid a hand on Red’s blaster skull, gently running a hand over all the scars that had elongated during the transformation. Red finished curling around him, his purring shaking Blue’s legs in little clattering sounds. Blue’s body almost crumpled as the fear left him. The adrenaline from his almost death, to thinking his boyfriend would eat him, to having a lap beast really took it out of a guy.

“So, are you in control or is it just the fact I’m your boyfriend?” 

Red eyes stared up at him, then tapped his tail on Blue’s face, sliding up his chin, a move Red did often with his hands when he had them.

“All you then.” Blue kissed Red’s nose. “Thanks for not eating me. No one would have suspected a thing.” He chuckled, Red huffing something similar to a laugh. Another lick, to his arm this time. “Gross!” Blue laughed though, dropping his arm in the snow.

Red picked up his head, facing into the woods. 

“What?” Blue stood up again when Red crouched down, moving into a pouncing position. Blue stood behind the curl of Red’s body. He peered into the darkness, snowy shadows as far as the eye could see, until he heard the crunch of snow.

“Red!” Blue fell on Red’s back when he whipped back. He clutched onto his hoodie, holding on for dear life while Red swiped at the intruders. Blue went to slide off, stopping at the sound from the trees.

THUMP

THUMP

THUMP

Why was something that big living here? A giant wolf rabbit hybrid stepped out the trees, all long sharp teeth and claws, but unusually fast. And it was looking straight at them.

Red growled and rolled his haunches. Blue laid forward on his back, securing himself quickly.

“Ready.”

Blue hend on tightly as Red skirted its legs. He snapped and bit at the thighs, earning a solid side swipe for the damage. Red barely managed to stop his body from crushing Blue. He sent a few sharp bones towards the eyes. The beast screeched.

It clawed at its face. It backed up into more claws, Red attacking wherever it left open, until it finally started to retreat. They needed to follow; if it went to the town square, it’d dust someone. Red bounded after it, hot on its heels until, in its pain and confusion, walked straight off a cliff.

“Oh no!” Blue clutched onto the Red for dear life.

Red managed to catch his claws in the monster, riding it down the steep snowy hill. He jumped off just before it hit the ground.

They tumbled off to the side, followed by a wave of dust.

Blue coughed, finally releasing his hold, fingers aching from how hard he’d forced them to hold on. When he finally stood up from the snow, he took a good look around.

They’d fallen off the edge of Snowdin. Steep rock faces stood between them and civilization. He looked over at Red, who’d made himself at home under a tree, unable to do much on his own. The red eyes followed Blue as he walked around the clearing they found themselves in, looking around for a climbable wall from here.

Nothing. Not a single way out, well, not for Blue. What did he have to work with?

“I can’t shortcut. I need you to get us out of this. You can’t change back until you’re out of magic.” Blue stated the facts, Red rolling his giant eyes, waiting for Blue to get to the point. “You can’t do direct magic in that form like the rest of us, I’m guessing.”

Red gave him an unimpressed look from where he curled underneath the tree.

“So we have to run you out of magic, preferably quickly, before the feral beasts of the forest go eat the town.”

Blue paced around. What would burn a lot of magic unintentionally?

Well, he knew one thing. But…they were outside, and Red was…a gaster beast…

But they also didn’t have enormous amounts of time. A day would make a huge difference in how their siblings fared against the threat. And it could be worse, it could be not his boyfriend he was stuck with.

Blue’s face got real bright, but he did lift his shaking nervous hands to his neckerchief. Red’s head looked over. He started to his feet when Blue dropped the blue cloth in the snow, dropping off his gloves. Blue pulled his shirt over his head, tossing it off to the side. Red’s snout bumped against his chest after he kicked off the boots.

“Don’t make me explain…”Blue got to his pants, stopping and rubbing over Red’s nose. “Though I guess I’d have to help you undress too.” 

Red backed up, sitting on his hind legs, tilting his head to try and tell Blue he didn’t get it, but Blue just dropped his pants, avoiding eye contact, folding the clothes and putting them to the side. Red growled when Blue went to get his pants off. They got cold much slower than most monsters, but that didn’t mean Red wanted to have a cold ass if he could help it.

“Red, I really need you to let me take off your pants.”

Red grumbled in complaint, but let Blue do as he pleased. He didn’t sit after they were off. Blue folded them too. He didn’t want them to be ruined, they’d need them when Red shortcut them out of this mess.

“You do know what I’m trying to do, right?”

Red shook his massive head. Blue looked away shyly.

“I know we’ve never purposely burned magic this way, but usually you think of this kind of thing before me.” Blue hesitantly dropped his boxers, still not meeting Red’s face. “If you don’t get it by now, I’m not convinced you’re actually Red at all-ahhhhhh.” He felt a wet tongue across his sternum.

Red was nosing all up on him now. 

“That’s more like you. Hah, mweeeeeeeh.” Red’s tongue was longer like this, going from licking at his chest to licking inside his ribcage and pelvis. It was haphazard. Red had very little control, unable to do much more than stimulate broadly. 

Blue took a peek over at Red’s body. No ecto yet.

“I really hope this works, or you’ll never let me live this down.” Blue got down on his knees. He ran his fingers over the front of his pelvis, catching all the bumps and curves. Red kept nuzzling and licking, careful to not disturb Blue’s arm.

“What would you say to me right now?” Blue closed his eyes. It was hard to get in the mood, naked in the snow and nervous, so he had to work himself up. Red was always so confident in the bedroom. Blue was confident everywhere else.

“Look at you working yourself up for me.” He could almost hear Red whispering it in his ear. “Gunna summon me something tasty?”

Blue panted as his fingers worked his sensitive pelvic crests, other hand working his tailbone at the same time. He moaned and opened his eyes. Red’s ectobody had finally appeared, sporting a longer and thicker member than he was used to. Blue blushed hard at the thoughts of what was to come. His own ectobody summoned under his hands, but Red’s head pushed him.

Blue fell onto his hands and knees, Red’s thick tongue working its way into him immediately.

“Reeeeeed! AAAAAAAH, so forceful mmmmmmm.” Blue wiggled his hips in time with the sloppy licks Red ate him out with. He usually covered his mouth when he got this loud, but there was no one but the forest to hear him.

“J-just gotta have you every w-way I can sweetheart. Come on my t-t-tongue?” Blue stuttered out what he thought his boyfriend would say. Red rumbled approvingly.

Blue reached a hand between his legs, and with a few skilled strokes, he whined out a release.

Red’s tongue didn’t leave him though. Blue huffed through the overstimulation until Red was satisfied, snout finally backing up from Blue’s shaky thighs. Blue only relaxed for as long as it took Red to line up with his thoroughly prepped magic. 

“Red…”He felt a giant claw near his body move a little closer in acknowledgement. “The whole point of this is to burn as much magic as possible so…” Blue moved onto his elbows, pressing himself just slightly back on the tip. Blue hummed with Red’s growl, feeling Red’s body position itself for the storm to come.

“Don’t stop until you can’t go anymore.” 

That first slide shook Blue to the marrow. Even as wet and stretched as he was, it was girthier than a tongue and feeling Red’s whole body move with the motion rocked Blue’s smaller body. Red’s control could snap right in the middle of this, but hopefully he’d be having enough fun to not kill the source of said fun.

“MMMMMMMM.” Red’s thrust slammed into the back of his pussy, Blue’s position the only thing keeping him in place. 

Red’s pace gradually picked up, Blue focusing on the rhythm of it, gradually building up to rocking back at the right points to feel the limits of his body stretch deeper. His moans got louder and louder. The imaginary voice in his head supplied Red’s commentary.

“That’s it baby! Let me hear how much you enjoy hav-Ahhhh-having me in your p-pussy!” His boyfriend had a filthy mouth, but Blue had let him do a lot with that mouth of his, and now Blue needed to hear him, it just wasn’t the same without his deep gravelly voice leading him through. Blue’s knees dug deeper into the snow. “Such a tight c-c-c-” -Blue had to fight to say it- “cunt you’ve made for me.”

Red’s growls shook his body. Red started pounding into him so fast, Blue screamed for it, body on fire and pushed over the edge. Once he felt the shudder of Red’s cock inside him, he stroked himself once and was gone. 

Blue spasmed and moaned, riding the wave. Red briefly pulled out, letting out the excess magic Blue had inside his little body. 

He almost stopped shaking before the tongue was in him again.

“R-r-r-red…”Blue whined desperately. His overstimulated body protested but couldn’t stop his oversized boyfriend. The tongue cleaned him out completely, before he felt Red gearing up to mount him again.

“AHHHHHH!” Red filled him in one go, already stretched open and wet.

Blue’s nerve endings burned, pleasure singed with discomfort in every bone in his body. Red kept pushing in and out, desperately taking Blue’s magic for all it could physically take. And as overwhelmed as he felt, Blue had been the one to tell Red not to stop until Red physically couldn’t continue. 

Red was filling him again. His body stretched and ached, Blue squeaking out a happy sigh when Red pulled out after a few minutes of slow fucking through the pleasure.

And then the damn tongue again.

Blue’s eyes pricked with tears. He’d never been so physically pushed to his mental limit. His vision felt fuzzy, and he felt like if he moved from this spot, he’d pass out. His traitorous mouth let out moans while he cried out his feelings. There was that cock again.

Everything blurred together. Thrusting and licking, alternating for what could’ve been days but was definitely at least a few _hours_ , nothing but overstimulation and being completely helpless to stop it. His mind had left him quite a few cycles ago, in a haze while his body took and took and _took_. Somewhere deep inside him, something broke, and when he felt that sinfully long tongue again, he leaned his hips back into it.

“ _God Red, get it all the way inside me_!” He felt deranged, sounded worse. His mind had given up on stopping it, deciding to ride the wave, no, tsunami as it engulfed him as best it could.

The tongue responded vigorously. Red’s snout dug into the cheeks of his ectobody, Blue drooling messily as his head laid in his arms above the snow. Blue keened at it twisting and turning against his sensitive walls.

“ _More, more, more! Ruin my body, **don’t stop until you can’t go anymore**_.”

Red didn’t do a gentle first thrust, immediately pounding into Blue’s body while he cried out for more. Blue’s soul was on fire. Every sensation in his whole body, every thought, was converging on a single moment, and when Red hit that brutal speed crescendo right before his release, Blue touched his over sensitive pussy.

“ _ **FUCK**_!” All of senses exploded, his mind slipped briefly into a serene peace, the eye of the storm, and with one ultimate burst of pleasure, his mind and body blipped off.

—-

“I’m surprised he got this exhausted without transforming.” Blue’s mind came to slowly, fluttering awake to a familiar ceiling. This was his room. His entire body ached, bringing back the memory of what put him into this state to begin with.

“Me too. He scared the hell outta me.” That was Red’s voice. Blue’s hand reached out for it without thinking. He felt Red’s hand take his instantly. “Blue?”

“I think so.” Blue chuckled, then coughed. Everything hurt.

“Thank goodness. We were worried sick.” He could hear his brother, but opening his eyes again seemed like a hassle. “Red and you vanished, but Red got you both back after a day. He’d been keeping you safe in the forest while you were unconscious.” So that’s what they were going with.

“Sorry for missing out on the fighting.” Blue hummed at Red gently rubbing his hand.

“Red came back just in time honestly. With his help, we got everything under control at the last moment. It all worked out.” His bro pat his head, but Blue winced. It still ached. “Sorry.”

“It’s all good Papy. I’ll be okay with a bit more rest. I’ll take a page from you and Red and be a lazybones for a while.” 

“Good.” Swap stood up, cracking his back. “Now that you’ve woken up at least once, I can go back to business. I’ll leave you with Red until you go back to sleep.” He smiled at Blue. “Don’t push it. Go back to bed. Love ya bro.”

“Love you too Papy.” Swap walked out, and Red let out a breath Blue hadn’t noticed he was holding.

“Why were you so nervous?” Blue smiled at him fondly. Red blushed for once.

“Blue, you were out for two days. I thought I put you in a coma, I at the very least hurt you pretty badly.” Red hunched in on himself. “So much for self control.” Blue squeezed the hand that held his.

“I didn’t exactly help. Not to mention, that’s never happened to anyone else, so now we know what’ll happen.” Blue gently tugged Red’s arm, scooting back a little. Red obliged his hurt boyfriend and climbed in.

“Doesn’t make it okay.” Red gently leaned their foreheads together. “Hurting you isn’t something I should ever do.” Blue clanked his mouth. The ache in his pelvis brought back memories to make him blush.

“I…liked it…”Blue averted his eyes, but they were so close that he could still see the look on Red’s face.

“You…liked it?” He looked confused, then surprised.

“Yeah.” Blue looked back at him, cheeks glowing. “I wouldn’t mind…doing that again…on purpose…”He could barely get the words out. His mind flashed to him begging for Red to have sex with him as a gaster beast, and he glowed brighter. Red laughed, eyes getting half lidded and his voice dropping down to that deep tone that set him off so much.

“Just gotta have you every way I can sweetheart.” Then he stuck out his tongue with a smirk, laughing while Blue buried his face right into Red’s jacket. Blue felt two arms pull him close. “You do a pretty good impression of me.”

“I hate you.” Blue pouted, not moving away from Red at all.

“Well I love you.” Red hummed into the little kiss he got in exchange.


End file.
